Exercise that induces substantial muscle glycogen depletion impairs subsequent anaerobic capacity

The purpose of this study was to develop a model of muscle glycogen depletion and to study the effect of this model on aerobic and anaerobic capacity of horses. The maximal rate of oxygen consumption (VO2max), maximal accumulated oxygen deficit (MAOD), muscle glycogen concentration and blood lactate concentration of 6 fit Standardbred horses were measured on 3 occasions 7 days apart (Trials 1, 2 and 3). Between Trials 2 and 3, strenuous exercise intended to deplete muscle glycogen was performed by exercising horses on the treadmill on 3 consecutive days. Strenuous exercise resulted in reduction of muscle glycogen concentration by at least 55% (from mean +/- s.e. 155.1 +/- 5.6 mmol/kg, wet weight, before Trial 2 to 55.4 +/- 5.5 mmol/kg before Trial 3; P < 0.05). VO2max was similar in Trials 2 and 3 (140.4 +/- 5.4 ml O2/kg bwt and 141.8 ml +/- 6.2 ml O2/kg, respectively). Run time to fatigue during a single high-speed exercise test (253.9 +/- 33.3 s and 153.8 +/- 16.4 s, P < 0.05), accumulated oxygen deficit (95 +/- 13.2 ml O2/kg and 35 +/- 13.9 ml O2/kg, P < 0.05) and blood lactate concentration at the end of the sprint (17 +/- 1.2 mmol/l and 10.5 +/- 1.1 mmol/l, P < 0.05) were less during Trial 3 than Trial 2. These data suggested that repeated strenuous exercise that causes muscle glycogen depletion results in impairment of anaerobic, but not aerobic, metabolism.